/**
 * Copyright 2010 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
package net.oauth.jsontoken.crypto;

import java.math.BigInteger;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;
import java.util.regex.Pattern;

import org.apache.commons.codec.binary.Base64;

/**
 * Class that can parse "magic key" RSA public key representations, which are of the form
 * RSA.<base64(big-endian(modulus))>.<base64(big-endian(exponent))>.
 */
public class MagicRsaPublicKey {

    private final PublicKey publicKey;

    /**
     * Public constructor.
     * @param magicKey the serialized key (of the form RSA.modulus.exponent).
     */
    public MagicRsaPublicKey(String magicKey) {
        this.publicKey = parseKey(magicKey);
    }

    /**
     * Returns the public key represented by the "magic" serialized key.
     */
    public PublicKey getKey() {
        return publicKey;
    }

    private static PublicKey parseKey(String magicKey) {
        String[] pieces = magicKey.split(Pattern.quote("."));
        if (pieces.length != 3) {
            throw new IllegalStateException("not a valid magic key: " + magicKey);
        }

        if (!pieces[0].equals("RSA")) {
            throw new IllegalStateException("unkown key type for magic key: " + pieces[0]);
        }

        String modulusString = pieces[1];
        String exponentString = pieces[2];

        byte[] modulusBytes = Base64.decodeBase64(modulusString);
        byte[] exponentBytes = Base64.decodeBase64(exponentString);

        BigInteger modulus = new BigInteger(modulusBytes);
        BigInteger exponent = new BigInteger(exponentBytes);

        RSAPublicKeySpec spec = new RSAPublicKeySpec(modulus, exponent);
        KeyFactory fac;
        try {
            fac = KeyFactory.getInstance("RSA");
        }
        catch (NoSuchAlgorithmException e) {
            throw new RuntimeException("RSA key factory missing on platform", e);
        }
        try {
            return fac.generatePublic(spec);
        }
        catch (InvalidKeySpecException e) {
            throw new IllegalStateException("bad key in descripor doc: " + magicKey, e);
        }
    }
}
